Project Summary The Under-5 Mortality Rate (U5MR) is a key and widely-used indicator of child health, but it conceals important information about how this mortality is distributed by age. For better understanding and monitoring of child health, it is critical to examine how the risk of death varies within the 0-5 age range. This includes age breakdowns beyond the standard cut-off points of 28 days (for neonatal mortality) and 1 year (for infant mortality). In many populations, however, the age pattern of under-5 mortality is not well known. Less- developed countries, in particular, lack the high-quality detailed vital registration information necessary for the analysis of such age patterns. Sample surveys collecting retrospective birth histories do not satisfactorily fill this gap, because they are subject to systematic biases that are particularly consequential for estimating age patterns. This makes the need for high-quality information on age patterns of under-5 mortality even more critical, because regularities in these age patterns can be used as a powerful tool for evaluating and correcting data when sources are deficient. In the parent project of this competing revision application, we are improving our understanding of age patterns of under-5 mortality by: gathering the largest database to date on high- quality global mortality information by detailed age (by days, weeks, months, and years of age) from birth until age 5, by sex; developing models summarizing regularities about how under-5 mortality is distributed by detailed age in human populations; using these models for evaluating and correcting under-5 mortality information by detailed age in less-developed countries; addressing specific substantive questions about how and why age patterns of under-5 mortality vary by sex, time, and place, with important programmatic implications. The goal of this competing revision project is to expand the aims of the parent project by integrating new and unique data, allowing us significantly increase the scope and application of the models developed in the parent project. Our analysis of Mozambique?s Countrywide Mortality Surveillance for Action (COMSA) will provide new information for developing a model that better takes into account the specificity of the African under-five mortality pattern. Our analysis of individual-level vital registration data from Egypt, Russia and Turkey will provide new applications of the models for indirect estimation of detailed mortality from birth to age 5. The integration of these new data will further the overarching goal of the parent project, which is to address specific substantive questions about how and why age patterns of under-5 mortality vary by sex, time and place. The data gathered as part of this competing revision will be included in the publicly- available online database which we are building as part of the parent project. This global database will allow the research community to easily access high-quality primary information on under-5 mortality by detailed age for their own research needs, and for further validation and replication of the project?s results.